The displaying of information for the driver of a motor vehicle, which must be currently provided to the driver, is becoming more and more important as the increasing internal networking (GPS, map data, current traffic situation, . . . ) and the installation of more and more sensors into the vehicle cause considerable amounts of data incurring for evaluation. Therefore, a display should be carried out in such a way that the driver can still keep an eye on the current traffic situation. HUD systems have proved to be particularly favorable for these tasks.
There are HUD systems with LCD displays for image generation and systems in which a micro display, for example comprising a Digital Micromirror Device (DMD), is imaged onto an intermediate image plane. This intermediate image can then be made available to the driver using a further optical system.
The design of the entire imaging channel (imager, intermediate image, virtual image for the driver) requires a tilting of the generated intermediate image in order to avoid reflections from the sun or other intense light sources. In order to provide an efficient integrated system, however, the projection system from the imager to the intermediate image requires a tilting of the intermediate image plane. Usually, such a tilting results in a plane which is inclined by at least 10 degrees to a main plane lying perpendicular to the beam axis. The beam axis as well as the tilting axis are each passing through the image center and intersecting each other perpendicularly.
In order to realize such a tilting of the intermediate image plane, different approaches are found in the state of the art. For example, such a tilting can be achieved by applying the Scheimpflug principle taking into account the so-called Scheimpflug condition. This means that an object plane with maximum sharpness is imaged in an image plane when the object plane, the main plane of the objective and the image plane intersect in a common straight line. Thus, in result of a corresponding tilting of the object (imager) and image (intermediate image plane), a tilted image with maximum sharpness can be realized. However, this option has the disadvantage that often a considerable distortion of the image field occurs. As a result, particularly in the case of high-resolution imagers with high information density, artifacts which are clearly visible can occur. The distortion has a further effect on a different resolution in the different image regions and leads to a strong inhomogeneity in the brightness of the image field. A further possibility for tilting the intermediate image plane is an off-axial use of the applied imaging optics, so that a corresponding tilting is realized especially for the center of the used image field. This option, however, has the disadvantage that the optics therefore have to be designed for a significantly larger image field and thus the projection system becomes considerably more expensive and heavier than necessary for purely optical reasons.
The objective problem of the invention is therefore to provide an improved projection system for display applications which avoids or at least significantly reduces one or more of the problems narrated in the state of the art. In particular, an objective problem of the invention is to provide a projection system from the imager to the intermediate image for a HUD with a tilted intermediate image plane with, compared to state of the art, significantly reduced image field distortion and a dimensioning of the imaging optics adapted to the image field size for light weight and low costs.